This online dictionary is very annoying (lots of ads and bugs) but is very useful. Enter a word, and it will give you many translations of that word and variations of that word. Importantly, the translations are HUMAN translations pulled from their database.

About.com's cluttered layout always annoys me, but their Japanese Language section is worth going through the clutter. It contains tons of lessons on grammar, vocabulary, expressions, and etiquette and is quite interesting to read.

The wiki article above, plus the wiki articles it links to, pretty much explains everything about sentence formation. However, it's written in a straight-forward and non-humorous matter rather than for a student who's trying to have fun learning Japanese (which is me).

Like the wiki link above, this site covers pretty much everything about sentence formation. However, it's written with a more "human feel" that's much more interesting to read. Additionally, the topics are organized and introduced with a student learner in mind whereas the wiki is more for reference and looking things up.

The exact opposite of Google Translate, Yahoo! Answers gives you 100% human translations. Of course, humans are imperfect, and the answers you get are only as good as the humans that give the answers (that's a weird sentence).